This application relates to a new dry technique for selectively photoetching a desired semiconductor material in the presence of other semiconductor materials. In a preferred embodiment, all materials have III-V compositions.
Device structures involving layers of binary, ternary, and quaternary compound semiconductors have become important in current device technology. A number of different ternary semiconductor systems are already in common use in devices: Al.sub.1-x Ga.sub.x As/GaAs, GaAs.sub.1-x P.sub.x /GaAs, GaAs.sub.1-x P.sub.x /GaP, and In.sub.1-x Ga.sub.x As have been extensively employed. Different material systems may prove equally important in the future. Some of these layered structures involve layers of materials, e.g., ternary or quaternary materials differing only in the relative amounts of the constituent elements. For device applications, it would be very desirable to have composition-selective dry etching techniques that would discriminate between such similar materials. Unfortunately, many systems exhibit little or no change in chemical reactivity with composition change. Current dry etching techniques offer selectivity between GaAs and Al.sub.1-x Ga.sub.x As based on different chemical reactivities, but not between Al.sub.1-x Ga.sub.x As materials of different relative Al and Ga compositions. (K. Hikosaka, et al, Jpn. J. Appl. Phys. 20, L847 (1981). The situation is even worse for the GaAs/GaAs.sub.1-x P.sub.x /GaP system, since the chemistries of GaAs and GaP are so similar.
Conventional dry etching processes for semiconductors, e.g., of the III-V-group involve Cl.sup.. or Cl- as the gas phase, "dry" reactant (G. Smolinsky et al, "Plasma Etching of III-V Compound Semiconductor Materials and Their Oxides", J. Vac. Sci. Technol. 18 (1981), 12-16). In the absence of light, these processes often do not exhibit significant or useful selectivity among III-V or other materials. This is especially the case when materials differ primarily only slightly in composition and thus have essentially the same chemical reactivity.
Dopant, concentration dependent, wet etching based on variations in sample bias voltage have been observed for Si (W. Kern, "Chemical Etching of Silicon, Germanium, Gallium Arsenide, and Gallium Phosphide", RCA Review 39 (1978) 278-308). For Si, a difference of two orders of magnitude in dopant concentration is required to produce useful differences in etch rate. Dopant concentration dependent wet etching of n-GaAs also has been observed (P.D. Greene, "Preferential Photoelectrochemical Dissolution of n-GaAs in Fe(III)-based Etches", Proc. 6th Int. Symp. on Gallium Arsenide and Related Compounds, Edinburgh, Sept. 20-22, 1976, p. 141-149). See also Kerr, supra. Voltage control of the rate of a wet etching process for n-GaAs has been reported (H. J. Hoffman et al., "Voltage-controlled Photoetching of GaAs", Appl. Phys. Lett. 38 (1981) 564-566).
Several photochemical wet etching processes have also been identified which produce preferential etching of n-type materials (R. W. Haisty, "Photoetching and Plating of Gallium Arsenide", J. Electrochem. Soc. 108, 790-4 (1961); F. Kuhn-Kuhnenfeld, "Selective Photoetching of Gallium Arsenide", J. Electrochem. Soc. 119, 1063-8 (1972); and R. M. Osgood, Jr., "Localized Laser Etching of Compound Semiconductors in Aqueous Solution", Appl. Phys. Lett. 40, 391-3 (1982)). A photochemical preferential wet etchant for p-GaP has been reported (W. H. Hackett, Jr., et al., "A Scanning Electron Microscope Investigation of Etching Phenomena in GaP Electroluminescent Diodes", J. Electrochem. Soc. 119, 973-6 (1972)).
However, in none of these disclosures was there any suggestion of a photochemical dry etching selectivity based solely on small compositional differences.
Such selective dry etching processes would be extremely useful in the fabrication of semiconductor devices (especially III-V devices where the problem is most severe) since dry processes offer several well-known advantages over wet processes for commercial production (R. G. Poulsen, "Plasma Etching in Integrated Circuit Manufacture-A Review", J. Vac. Sci. Technol. 14, 266-74 (1977)).
Many conventional processes are known for etching a wide variety of substrates and for photopatterning substrates using photoresists and other techniques. However, the details of the etching steps of these methods are unrelated to the problem of selectively etching semiconductor substrates according to composition. As a sampling, see, e.g., U.S. Pat. Nos. 4,478,677; 4,414,059; 4,320,191; and 4,252,891. Similarly unrelated is U.S. Pat. No. 4,404,072 disclosing a photoelectrochemical wet etching technique. No selective etching between different semiconductor materials is reported. Similar disclosures involving details of etching techniques which do not achieve any selection between semiconductor materials also include U.S. Pat. Nos. 4,351,706; 4,454,004; 3,364,087; 4,326,911; and 4,331,504.
Although some of the very basic aspects of dry etching processes are known, e.g., the relationship between bandgap and photon energies in photochemical etching of Si by XeF.sub.2 (Houle - "Non-thermal Effects in Laser-Enhanced Etching of Si by XeF.sub.2 ", Chem. Phys. Lett. 95 (1983) 5-8), selective dry etching techniques have not been disclosed heretofore.
All of the disclosures cited above are incorporated by reference herein.